Binary Tree Preorder Traversal

Given a binary tree, return the preorder traversal of its nodes' values.

For example:

Given binary tree {1,#,2,3},

return [1,2,3].

Note: Recursive solution is trivial, could you do it iteratively?

java code ：recursive solution:

/** 
 * Definition for binary tree 
 * public class TreeNode { 
 *     int val; 
 *     TreeNode left; 
 *     TreeNode right; 
 *     TreeNode(int x) { val = x; } 
 * } 
 */  
public class Solution {  
    public ArrayList<Integer> preorderTraversal(TreeNode root) {  
        // IMPORTANT: Please reset any member data you declared, as  
        // the same Solution instance will be reused for each test case.  
        ArrayList<Integer> res = new ArrayList<Integer>();  
         if(root == null)  
             return res;  
         pre(res,root);  
         return res;  
    }  
    public void pre(ArrayList<Integer> res, TreeNode root)  
     {  
         res.add(root.val);  
         if(root.left != null)  
             pre(res, root.left);  
         if(root.right != null)  
             pre(res, root.right);  
     }  
}

iteratively solution:

/** 
 * Definition for binary tree 
 * public class TreeNode { 
 *     int val; 
 *     TreeNode left; 
 *     TreeNode right; 
 *     TreeNode(int x) { val = x; } 
 * } 
 */  
public class Solution {  
    public ArrayList<Integer> preorderTraversal(TreeNode root) {  
        // IMPORTANT: Please reset any member data you declared, as  
        // the same Solution instance will be reused for each test case.  
        ArrayList<Integer> res = new ArrayList<Integer>();  
         if(root == null)  
             return res;  
         Stack<TreeNode> st = new Stack<TreeNode>();  
         st.push(root);  
         while(!st.isEmpty())  
         {  
             TreeNode cur = st.peek();  
             res.add(cur.val);  
             st.pop();  
             if(cur.right != null)  
                 st.push(cur.right);  
             if(cur.left != null)  
                 st.push(cur.left);  
         }  
         st = null;  
         return res;  
    }  
      
}

补充：软件开发 , Java ,